1. Field of the Invention
The present invention relates to an electric double layer capacitor, particularly an electric double layer capacitor suitable for an application which requires a high output.
2. Discussion of Background
The electric double layer capacitor is based on a principle to store electric charge in an electric double layer formed at the interface between a polarizable electrode and an electrolyte. It is possible to carry out quick charging and discharging with a larger current as compared with a cell, and application to the field of energy has been actively studied in recent years. For example, a capacitor which provides a large capacitance and a high output has been proposed in JP-A-8-45793. As a specific application, an attention has been drawn to the application to a electric car or a hybrid car. For such an application, it is required to develop a power source operating at a high output density of 500 W/kg in near future, and 1500 W/kg ultimately, as target values set by the Department of Energy, U.S.A. (A.F. Burke et. al., Material Characteristics and the Performance of Electrochemical Capacitors for Electric/hybrid Vehicle Applications, Materials Research Society Spring Meeting, San Francisco, Calif., 1995.4.17-21).
The specific gravity of an electric double layer capacitor cell may depend upon the structure or the housing. However, when an unnecessary space is removed and the housing is made light, it is usually from about 1.4 to about 1.8 g/cm3. Accordingly, by calculating the output density as the output per volume, 500 W/kg, 1000 W/kg and 1500 W/kg correspond to about 800 W/L, about 1600 W/L and about 2400 W/L, respectively. Heretofore, many reports have been made about the initial performance of an electric double layer capacitor capable of being used at such a high output density. However, for an application to an electric car or the like, charging and discharging cycle reliability such as durability against 50,000 cycles of charging and discharging at large current is required, and an electric double layer capacitor having not only a high output density but also such a high reliability has not yet been obtained.
As an electrolyte for an electric double layer capacitor, an organic electrolyte and an aqueous electrolyte are available. However, an attention has been drawn to an electric double layer capacitor employing an organic electrolyte, as the operating voltage is high, and the energy density in the charged state can be made high. In the case where an organic electrolyte is employed, in order to improve the capacitance per unit area of an electric double layer capacitor (hereinafter referred to as capacitance density), a carbon material such as activated carbon is used. If water is present in the cell of an electric double layer capacitor, the performance tends to deteriorate due to electrolysis of the water. Accordingly, the electrode is generally subjected to heat treatment under reduced pressure to sufficiently remove water.
As a method of preparing the electrode, a method may, for example, be mentioned, wherein a carbon fine powder is dispersed in a solution having a binder such as carboxymethylcellulose dissolved in a solvent, to form a slurry, which is then coated on a current collector, followed by drying to form an electrode layer on the current collector. However, with this method, the bonding strength between the electrode and the current corrector is weak. Further, since heat resistance of the binder is not adequate, the heat treatment can not be applied to the electrode at such a high temperature that the impurities in the electrode such as water can be adequately removed. Further, with this method, for example, in a case where the electrode is made a thick film of at least 60 xcexcm, it is difficult to uniformly remove the solvent from the slurry by drying, and it is difficult to form an electrode layer which provides a high strength, a low resistance, a high density and a high capacitance on the current collector with high productivity.
Further, a method has been proposed, which comprises preliminarily molding a kneaded material comprising a carbonaceous material such as activated carbon, a binder such as polytetrafluoroethylene (hereinafter referred to as PTFE) and a liquid lubricant, and forming it into a sheet by stretching or rolling to obtain an electrode (JP-A-63-107011, JP-A-2-235320). According to the method, as PTFE is formed into fibers, the ion conduction is not likely to be inhibited, and the carbonaceous material is filled highly densely. With regard to the bonding of the electrode and the current collector, by bonding them by means of an electroconductive adhesive layer, the bonding strength can be made high and the electrical contact resistance can be made small. Further, as PTFE is thermally and electrochemically stable, an electric double layer capacitor which provides high reliability, a high capacitance and a low resistance, can be constituted.
However, in an application wherein it is required to carry out charging and discharging with a large current, it is necessary to further decrease the resistance of the electrode, and it is effective to make the electrode thin. However, in the above electrode, PTFE is randomly formed into fibers by kneading, and a part of PTFE is formed into fibers and the rest is not. Therefore, when forming the electrode into a sheet having, for example, a thickness of at most 200 xcexcm, the surface tends to be irregular, and holes are likely to be formed. Therefore, the capacitance density of the electric double layer capacitor can not be made large, and the internal resistance can not be sufficiently decreased.
To solve such problems, the present inventors have proposed a method which comprises extruding a mixture comprising a carbonaceous material, PTFE and a processing aid by screw extrusion, followed by rolling, to obtain a porous electrode sheet having a high strength and a thickness of at most 200 xcexcm (JP-10-19758). According to the method, an electrode sheet can be continuously obtained industrially with high productivity.
The present inventors have established a technique to easily control the thickness of the electrode sheet of high strength by the above-mentioned method for producing an electrode. It is an object of the present invention to provide an electric double layer capacitor for power source, particularly an electric double layer capacitor source, particularly an electric double layer capacitor which provides a high output density and a high energy density and which is excellent in charging and discharging cycle durability, by utilizing the technique. The present invention provides an electric double layer capacitor comprising a positive electrode assembly and a negative electrode assembly, each electrode assembly having a porous layer comprising a carbonaceous powder and a fluorine-containing polymer formed on at least one side of an aluminum foil current collector, the positive electrode assembly and the negative electrode assembly being disposed so that the porous layer of the positive electrode assembly and the porous layer of the negative electrode assembly face each other with a separator interposed therebetween to form an element, the element being impregnated with a non-aqueous electrolyte and accommodated in a sealed container, wherein the thickness of the porous layer is from 80 to 200 xcexcm, the thickness of the aluminum foil current collector is from 20 to 80 xcexcm, the thickness of the separator is from 30 to 170 xcexcm, and the density of the porous layer is from 0.50 to 0.80 g/cm3.